Methods and systems for controlling turbocompressors

ABSTRACT

A method for regulating a turbocompressor to prevent surge, is described. The method comprises the following steps: providing at least one surge limit line of turbocompressor; continuously determining an actual value of a corrected speed of the turbocompressor; continuously determining at least a maximum admissible pressure ratio on the surge limit line, corresponding to the actual value of the corrected speed; continuously determining an actual pressure ratio; acting upon an antisurge arrangement, if the actual pressure ratio is equal to or higher than the maximum admissible pressure ratio.

BACKGROUND

The present disclosure relates to compressor systems and more particularly to turbocompressor systems including axial and/or centrifugal compressors for processing a gas flow. The subject matter of the present disclosure concerns methods and systems for controlling the compressors arrangement to prevent out of operating envelope phenomena like surge and other undesirable operating conditions.

DESCRIPTION OF THE RELATED ART

Turbocompressors are work-absorbing turbomachines used to boost the pressure of a working gaseous flow. The pressure of the working fluid is increased by adding kinetic energy to a continuous flow of working fluid through rotation of a rotor supporting one or more impellers and/or one or more sets of blades in circular arrangements. Turbocompressors are frequently used in pipeline transportation of natural gas, for example to move gas from a production site to a consumer location, in gas and oil applications, refrigeration systems, gas turbines, and other applications.

The flow of fluid through the turbocompressor can be affected by various conditions leading to unstable operations which can result in serious damages of the turbomachine.

Compressor surge occurs when the pressure of a working fluid flowing through the compressors increases beyond a maximum allowable output pressure and/or if the flow rate drops beyond a minimum limit.

In general a surge phenomenon occurs when the compressor cannot add enough energy to the working fluid in order to overcome the system resistance, i.e. the head drop across the system, a situation which results in a rapid flow and discharge pressure decrease. The surge may be accompanied by high vibrations, temperature increase and rapid changes in the axial thrust on the bearings of the compressor shaft. These phenomena can severely damage the compressor and also the components of the system connected to the compressor, such as valves and piping.

Other undesirable operating conditions can arise during operation of the turbocompressor. More particularly, choke (also sometimes named stonewall) is a condition at which increased flow results in a rapid decrease in head, i.e. compression ratio a flow is increased. Operating at a very high flow rate has negative effects on the compressor performance and can result in compressor damages.

To prevent surge and choke phenomena to arise, control systems have been developed and are currently used in turbocompressor installations

FIG. 1 schematically illustrates an exemplary embodiment of a system 1, comprised of a turbocompressor 3 driven into rotation by a prime mover 5, for example an electric motor, a gas or steam turbine, or the like. Reference number 7 indicates a suction line, where from the working fluid is fed to the suction or inlet side of the turbocompressor 3. Reference number 9 designates the delivery pipe, where through the compressed fluid is delivered from the discharge side of the compressor 3.

FIG. 2 schematically illustrates a compressor performance map, typically a compressor performance map of an axial compressor. The performance map shows the pressure ratio along the vertical axis and the inlet volumetric flow reported on the horizontal axis. The inlet flow is indicated with the letter Q. Depending upon the operating conditions of the compressor, for example the rotary speed (rpm), a plurality of expected performance curves can be reported in the performance map. Each curve can correspond to a different compressor rotary speed. For a given compressor setup, therefore, a family of performance curves can be reported on the performance map. Similar curve families can be drawn for different setup or operating conditions of the turbocompressor, e.g. for different positions of variable stator vanes (VSVs), the turbocompressor can be provided with. Each performance curve is limited by a surge point, i.e. a point where the pressure ratio and the gas flow through the compressor have achieved a value, beyond which surge phenomena will be generated. Each performance curve is further limited by a choke point, beyond which choke phenomena arise. The line SLL is the so called Surge Limit Line, formed by the surge points of the various performance curves reported on the performance map. The line CLL is the choke limit line, formed by the choke points. The SLL and CLL lines define an envelope, i.e. a portion of the performance map, within which the operating point of the compressor is maintained to ensure stable operating conditions of the turbocompressor and prevent both surge as well as choking conditions.

The SLL and CLL thus represent the limit of operation of the turbocompressor, beyond which the turbocompressor shall not be operated to prevent the risk of surge and choke phenomena. Known compressor systems are comprised of control devices and arrangements to control the turbocompressor so that it will constantly operate inside the stability area of the performance map, i.e. between the surge limit line SLL and the choke limit line CLL.

In the diagrammatic representation of FIG. 1 a control unit 11 is connected to various instrumentalities surrounding the turbocompressor to determine the operating conditions of the turbomachine and provide antisurge control and antichoke control for preventing surge and choke phenomena from arising.

More particularly, as shown in FIG. 1, the control unit 11 is connected to a flow measuring device, also called flow element 13 that is designed and configured to determine the inlet volume flow rate of the turbocompressor 3. A temperature sensor at the inlet side or suction side provides a temperature value Ts and pressure sensors provide the delivery pressure value Pd and suction pressure value Ps or directly the compression ratio Pd/Ps.

Based on the input data the control unit 11 is capable of determining inlet volume flow rate and the pressure ratio at each and every instant of operation of the turbocompressor 3. These two parameters define the operating point on the compressor performance map of FIG. 2. As additional parameter the rotary speed N (rpm) of the compressor can be provided, so that the correct operating curve can be selected to determine the actual position of the compressor operating point in the performance map. If the operating point moves close to the surge limit line SLL, the surge control system acts upon an antisurge bypass valve 15. The valve 15 is arranged on a bypass line 17 connecting the delivery side and the suction side of the compressor 3. A fraction of the working fluid delivered by the turbocompressor 3 can be recirculated through the antisurge valve 15, if required, to prevent surge phenomena. When the delivery pressure increases so that the operating point approaches surge limit line SLL, the antisurge control arrangement opens the antisurge bypass valve 15 so that the flow rate through the compressor can increase and the delivery pressure can decrease.

Before being recirculated through the antisurge valve 15 the working fluid can be cooled in a heat exchanger 19.

In some embodiments, the surge control arrangement can provide for a bleeding line, along which the antisurge valve is arranged and which is designed to discharge the process gas in the environment, if the nature of the gas so permit.

Choking of the compressor can be prevented by closing an antichoke control valve arranged along the suction line 7, or along discharge line upstream or downstream of the turbocompressor 3.

The actual known solutions require a flow element 13 for determining the operating point of the compressor for the purpose of preventing surging phenomena. In some application the flow element 13 can be cumbersome and requires a relatively long pipe upstream and downstream thereof, in order to provide a correct measurement of the inlet flow rate. Providing measuring elements or devices at the inlet side or suction side of turbocompressor, in particular air turbocompressor can be difficult.

SUMMARY OF THE INVENTION

The subject matter disclosed herein concerns an improved method and apparatus for providing antisurge control of a compression system comprised of at least one compressor. In some embodiments, the method and apparatus provide antisurge and/or antichoke control of the compressor.

In some embodiments, at least one operating envelope is defined, the compressor being controlled so that the operating point thereof falls within the operating envelope. Action is taken, if the operating point falls outside or on the boundaries of the operating envelope or if in an embodiment, the operating point approaches the boundaries of the envelope. The operating envelope is defined based in a performance map based on two operating parameters of the compressor: a corrected speed of the compressor and a pressure ratio. The pressure ratio is the ratio between the delivery pressure and the suction pressure of the compressor. The corrected speed is defined as a function of the suction temperature of the gas being processed by the compressor and the rotary speed of the compressor. The corrected speed is thus proportional to the ratio

$\frac{N}{\sqrt{Ts}}$

wherein:

-   -   Ts is a processing-fluid temperature at compressor inlet and     -   N is the rotary speed of the compressor.

The corrected speed is defined by the above mentioned ratio if the gas composition is constant. The method disclosed herein, therefore, is suitable for antisurge/antichoke control of compressors processing a gas having a known and constant composition, e.g. carbon dioxide and the like.

The operating envelope can be bounded by a suction limit line, a choke limit line, as well as by a maximum admissible corrected speed and by a minimum admissible corrected speed.

If the compressor is provided with movable inlet guide vanes, i.e. with variable stator vanes, an operating envelope can be defined for each position of the variable stator vanes. Thus, according to some, a plurality of operating envelopes are defined in a corrected speed versus compression ratio diagram or performance map. Depending upon the actual position of the variable stator vanes, the corresponding operating envelope is selected for antisurge and/or antichoke control. Since the position of the variable stator vanes can vary in a continuous manner, according to some embodiments, a limited number of operating envelopes are determined, corresponding to a limited number of different positions of the variable stator vanes. If the actual position of the variable stator vanes is different from those for which an envelope has been determined and the relevant data thereof stored for control purposes, a new intermediate operating envelope is calculated, e.g. by interpolating the data of the two nearest operating envelopes, for which data are available.

According to some embodiments, therefore, a method for regulating a turbocompressor to prevent surge is provided, comprising the following steps: providing at least one surge limit line and/or at least one choke line for at least one operating condition of the turbocompressor; determining continuously an operating point of the compressor measuring a processing gas temperature at compressor inlet, the rotary speed of the compressor, a delivery and suction pressure value; continuously determining an actual value of a corrected speed of the turbocompressor, being the correct speed proportional to the ratio

$\frac{N}{\sqrt{Ts}};$

continuously determining at least a maximum admissible pressure ratio on the surge limit line and/or at least a minimum admissible pressure ratio on the choke line, corresponding to the actual value of the corrected speed; continuously determining an actual pressure ratio, equal to the ratio between delivery pressure and suction pressure; acting upon an antisurge arrangement to recirculate a fraction of the compressed gas in the compressor through the suction line, if the actual pressure ratio is equal to or higher than the maximum admissible pressure ratio or equal to or lower than the minimum admissible pressure ratio.

In the context of the present disclosure and enclosed claims, the term “continuously determining” a parameter also encompasses determining in embodiments, the parameter at constant or variable time intervals during continued operation of the compressor.

In embodiments, the surge limit line defines, along with a choke limit line and maximum and minimum admissible corrected-speed lines, the operating envelope, within which the operating point of the compressor will be maintained.

If the operating point of the compressor is approaching the surge limit line, an antisurge arrangement can be acted upon. The antisurge arrangement can be any arrangement known from the art. Surge is prevented by opening an antisurge bypass valve. In a particular embodiment, if the process gas is air, surge can be prevented by venting or bleeding a fraction of the compressor delivery flow in the environment. In both cases, the delivery flow is increased, thus shifting the operating point of the compressor away from the surge limit line.

The method can also include the preceding step of detecting the kind of gas or the gas composition entering in the turbocompressor.

Since the correct speed is related to the gas composition or type, the gas detection or prediction could allow an online continuous setting of the method.

In particular, when the working gas has a composition not constant over time, the method requires information about the gas worked by the turbocompressor. For obtaining this information could be used any gas detector known in the art, for example a process gas chromatograph.

The apparatus can also comprises a suitable database for containing the operating envelopes associated to respective gasses.

If the operating point of the compressor approaches the upper corrected speed limit or the lower corrected speed limit, action can be taken to reduce or increase the rotary speed of the compressor, respectively.

If the operating point of the compressor is approaching the choke limit line, an antichoke arrangement can be acted upon. In an embodiment, the arrangement can be any arrangement known in the art. For example, an antichoke valve can be closed.

In some embodiments, variable stator vanes, i.e. movable inlet guide vanes can be provided at the suction side of the compressor. The variable stator vanes can be used as a control means to prevent the compressor operating point from approaching or moving beyond the lines delimiting the operating envelope. Choke can e.g. be prevented by reducing the inlet cross section and thus the inlet flow of the compressed gas.

The variable stator vanes can be acted upon also to prevent the compressor operating point to move above the upper corrected speed limit or to drop under the lower corrected speed limit.

According to a further aspect, the subject matter disclosed herein concerns an apparatus for providing an antisurge and/or antichoke control for a compression system comprising at least one compressor, the apparatus performing the control method in an embodiment is defined above. According to yet a further aspect, the subject matter disclosed herein concerns a compression system comprised of at least one compressor and the apparatus in an embodiment for antisurge and/or antichoke control.

According to a further aspect, the subject matter disclosed herein concerns a method for regulating a turbocompressor, comprising the following steps: determining at least one compressor operating envelope on a corrected speed versus pressure ratio diagram or performance map, the operating envelope being bounded by a choke limit line, a surge limit line, a maximum admissible corrected speed line and minimum admissible corrected speed line; continuously determining an operative point of the turbocompressor on the corrected speed versus pressure ratio diagram; determining whether the operative point is contained in the operating envelope; acting upon an actuating system to modify at least one operating parameter of the turbocompressor, if the operative point of the turbocompressor is not within the operating envelope. Since, the present can avoid the dispersion of gas in the environment, it's particularly suitable for polluting gasses.

Features and embodiments are disclosed here below and are further set forth in the appended claims, which form an integral part of the present description. The above brief description sets forth features of the various embodiments of the present invention in order that the detailed description that follows may be better understood and in order that the present contributions to the art may be better appreciated. There are, of course, other features of the invention that will be described hereinafter and which will be set forth in the appended claims. In this respect, before explaining several embodiments of the invention in details, it is understood that the various embodiments of the invention are not limited in their application to the details of the construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.

As such, those skilled in the art will appreciate that the conception, upon which the disclosure is based, may readily be utilized as a basis for designing other structures, methods, and/or systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 illustrates a schematic of a compressor system according to the current art;

FIG. 2 illustrates a compressor performance map currently used in antisurge and antichoke control systems;

FIG. 3 illustrates a schematic representation of a compressor system according to the present disclosure;

FIG. 4 illustrates a schematic representation similar to the one of FIG. 3 in a system comprising a turbocompressor with movable variable stator vanes;

FIG. 5 illustrates a compressor performance map according to the present disclosure showing one operating envelope;

FIG. 6 illustrates a compressor performance map showing two overlapping envelopes corresponding to two different positions of the movable inlet guide vanes or variable stator vanes of the turbocompressor;

FIG. 7 illustrates a flow diagram summarizing the control algorithm according to the present disclosure.

DETAILED DESCRIPTION

The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit the invention. Instead, the scope of embodiments of the present invention is defined by the appended claims.

Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase “in one embodiment” or “in an embodiment” or “in some embodiments” in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

FIG. 3 schematically illustrates a compressor system 20 embodying the subject matter disclosed herein. The compressor system 20 comprises a turbocompressor 21, for example a centrifugal or an axial turbocompressor. The turbocompressor 21 can be driven into rotation by a mover 23. In some embodiments the mover 23 can be an electric motor. In other embodiments the mover 23 can be a gas turbine, for example an aeroderivative gas turbine. In yet further embodiments different prime movers can be used, for example a steam turbine. A load coupling 25 connects the prime mover 23 with the turbocompressor 21. A speed manipulation device (not shown), for example a gear box, can be arranged between the prime mover 23 and turbocompressor 21.

A working gas is fed to the inlet or suction side of turbocompressor 21 through a suction line 25 and the compressed fluid is delivered from the delivery side of the compressor through a delivery line, or pressure line, 27. Non-return or check valves 29A and 29B can be arranged in the suction line and/or in the pressure line 27.

A heat exchanger 31 can be arranged on the pressure line 27 or, as shown in dotted lines at 31 x along a bypass line 33 connecting the pressure line 27 to the suction line 25. An antisurge, bypass valve 35 is arranged along the bypass line 33. The antisurge valve 35 is controlled by an antisurge control system 37 that will be described in more detail here below.

In a particular embodiment, e.g. when the working fluid is air or anyhow a fluid which can be discharged in the environment, the antisurge valve 35 can be arranged on a bleeding line discharging the working fluid directly in the atmosphere, even if a part is recirculated through the suction line 25.

To detect the working gas entering in the turbocompressor, the system could be equipped with an arrangement 50 for detecting the composition or kind of gas. This arrangement could be a more particular kind of gas chromatograph.

A control unit 39 is further provided in the system 20. The control unit 39 is interfaced with a temperature sensor 41 at the inlet or suction side of the turbocompressor 21, as well as with pressure sensors. The pressure sensors directly or indirectly provide a measure of the pressure ratio between the delivery side and the suction side of the compressor. By way of example, in the schematic of FIG. 3 a delivery side pressure sensor 43 provides a value Pd of the delivery pressure at the discharge side of the turbocompressor 21. A pressure sensor 45 at the inlet of the turbocompressor 21 provides a measure of the suction process Ps at the inlet of the turbocompressor 21. The pressure ratio can be calculated by the control unit 39, based on the two measured pressure values Pd, Ps. In other embodiments, the pressure ratio can be determined outside the control unit 39 and a pressure ratio value can be directly entered in the control unit 39.

A rotary speed sensor further provides a rotary speed value N (expressed e.g. in rpm) to the control unit 39.

Based on the operating parameters mentioned above, the control unit 39 is thus capable of calculating the pressure ratio of the compressor as well as the so called corrected speed of the compressor, defined as follows:

${Nc} = {\frac{N}{\sqrt{Ts}}*C\; 1}$

wherein

-   -   C1 is a function of temperature, pressure and gas composition     -   N is the rotary speed of the turbocompressor 21 and     -   Ts is the absolute temperature at the suction of the         turbocompressor 21.

The factor C1 is a function of gas composition and it is assumed constant if the gas has an invariable composition and T and P are in a restricted range.

In case of a gas having a known and constant composition the corrected speed can be simplified as follows:

${Nc} = \frac{N}{\sqrt{Ts}}$

The corrected speed can be used to define a compressor performance map, wherein the corrected speed is reported on one of the coordinates and the pressure ratio is reported on the other coordinate.

FIG. 5 schematically shows a performance map of this kind, wherein the corrected speed is reported on the vertical axis and the pressure ratio Pd/Ps is reported on the horizontal axis. On this performance map, a surge limit line SLL can be drawn. To prevent surging phenomena, the compressor 21 shall operate so that the operating point thereof on the performing map of FIG. 5 remains on the surge control line or on the left side thereof, so that the compressor will never operate on or beyond the surge limit line SLL.

On the same performing map of FIG. 5 a choke limit line CLL can also be drawn, which indicates the limit beyond which choking phenomena can occur. To operate the compressor 21 free of choking, the operating point of the compressor shall not move beyond the choke limit line CLL, on the left thereof.

On the performing map of FIG. 5 two further lines are drawn, namely a minimum admissible corrected speed line (Nc)min and a maximum admissible corrected speed line (Nc)max. The latter are straight lines parallel to the horizontal coordinate (abscissa) and represent respectively: the minimum admissible corrected speed below which the compressor shall not operate; and the maximum corrected speed, beyond which the turbocompressor shall operate.

The four lines defined above form an operating envelope OE. The compressor control system shall control the compressor so that the operative point thereof remains inside the operating envelope OE. In FIG. 5 an exemplary operating point labeled OP is indicated, corresponding to a corrected speed value Nc and a pressure ratio PR=Pd/Ps.

The control system is designed and arranged so that when the point OP moves towards the right and reaches the surge limit line SLL the operating conditions of the turbocompressor 21 are modified to bring the operating point OP back into the operating envelope OE. This can be obtained e.g. by opening the antisurge valve 35. When the operating point OP moves to the left until it achieves the choke limit line CLL, the control system will operate so as to modify the flow conditions bringing back the operating point inside the operating envelope OE. This can be done e.g. by acting upon an antichoke valve 47.

Moving below the minimum admissible corrected speed value (Nc)min is prevented by increasing the rotary speed of the compressor 21, if the operating point OP moves down reaching the (Nc)min line. Moving of the operating point above the maximum admissible corrected speed value (Nc)max is prevented by reducing the speed of rotation of turbocompressor 21 accordingly.

In the simplified schematic representation of FIG. 3 the turbocompressor 21 is not provided with movable inlet guide vanes or variable stator vanes (VSVs). These latter are usually provided in common turbocompressors to modify the geometry of the inlet cross section depending upon the operating conditions of the system. FIG. 4 represents the same system of FIG. 3, with the addition of variable stator vanes schematically shown at 51. The same reference numbers as in FIG. 3 indicates the same or corresponding components or parts, which will not be described again. In the system of FIG. 4 the control unit 35 further receives information on the actual position of the variable stator vanes of the turbocompressor 21. Reference VSV indicates the information concerning the actual position of the variable stator vanes during operation of the turbocompressor 21. The VSV position can be set by suitable actuators, not shown. The actuators can be controlled by the same control unit 37.

The antichoke valve 47 has been omitted from the schematic of FIG. 4, since choking can be prevented alternatively by acting upon the VSV. The latter are closed to reduce the inlet volume flow rate of the turbocompressor to avoid compressor choking, without necessarily using an antichoke valve.

In actual fact, for each possible position of the movable variable stator vanes 51 a different performance map and therefore a different operating envelope can be drawn. This is schematically represented in FIG. 6, wherein two different operating envelopes labeled OE1 and OE2 are represented. Each operating envelope is bounded by four curves which are defined in both instances in the same way as described above in connection with FIG. 5. Therefore, each operating envelope is bounded by a surge limit line SLL, a choke limit line CLL, a maximum admissible corrected speed line (Nc)max, and a minimum admissible corrected speed line (Nc)min.

As a matter of fact an indefinite number of operating envelopes can be provided, one for each position of the variable stator vanes. Data defining each operating envelope can be stored in a memory accessible by the control unit 37, and schematically shown in 38 in FIGS. 3 and 4. In practical embodiments, only a finite number of operating envelopes will be calculated and stored for example in the form of lookup tables or the like. During operation of the turbocompressor 21, the control unit 37 will use the operating envelope corresponding to the actual position of the variable stator vanes, if such envelope exists. If the actual position of the variable stator vanes is different from those for which an operating envelope has been stored in the control system, the control unit will calculate an operating envelope, for example by interpolation of the existing data, using the data corresponding to the two nearest VSV positions, for which the operating envelopes are available in the storage memory.

The operation of the system described so far will become clearer from the following description with reference to the above figures as well as referring to the flow chart of FIG. 7. In the latter the control process is represented as a sequence of steps. It shall be understood that in order to obtain a continuous control of the operating conditions of the compressor 21, the sequence of method steps represented in FIG. 7 will be repeated continuously and iteratively during operation of the system.

At start of the control process, the corrected speed Nc is determined. This is done by detecting the rotary speed N and the temperature Ts at the suction side of the turbocompressor 21. If the turbocompressor 21 is provided with variable stator vanes 51 as described in connection with FIG. 4, the VSV position is determined. Based on the data concerning the actual position of the variable stator vanes, the operating envelope is calculated, using data stored for example in the storage memory 38. As mentioned above, for some of the variable stator vane positions operating envelope data can be directly stored in the storage memory 38. For other intermediate positions the operating envelope can be calculated by for example interpolating the existing data.

Once the operating envelope has been determined, the maximum and minimum pressure ratio for the actual corrected speed Nc can be determined. This maximum and minimum ratios are indicated PRmax and PRmin in FIG. 5.

The actual operating point of the compressor is then determined based on the corrected speed Nc calculated as mentioned above and on the actual pressure ratio determined by the pressure sensors, which measure the delivery pressure Pd and the suction pressure Ps of the turbocompressor 21. The actual pressure ratio is indicated PR in FIG. 5.

At this point the control system has all the data required to act upon the antisurge and/or the antichoke arrangement to prevent choking or surging of the system. In some embodiments a surge parameter and a choke parameter can be calculated as follows. The surge parameter is defined as

$\frac{PRmax}{PR}$

wherein:

-   -   PRmax is the maximum admissible pressure ratio;     -   PR is the actual pressure ratio.     -   The choke parameter can be defined as follows:

$\frac{PRmin}{PR}$

wherein:

-   -   PRmin is the minimum admissible pressure ratio.

The surge parameter and the choke parameter can be used to generate control signals acting upon actuators controlling the antisurge valve 35 and the antichoke valve 47 and/or the VSV 51. If the surge parameter becomes equal to 1, i.e. the compressor operating point moves on the surge control line, the actuator of antisurge valve 35 will be acted upon to at least partly open the antisurge valve 35 on the bypass line 33. Working gas is recirculated from the delivery side to the suction side of the compressor to move the operating point OP back into the operating envelope OE. If the choke parameter becomes equal to 1, the antichoke valve 47 on the suction line 25 will be partly closed to reduce the suction flow rate and move the operating point of the compressor back inside the operating envelope OE.

The actual corrected speed value Nc being known by the control system, also correction of the rotary speed N of the turbocompressor 21 can be performed if required, in order to prevent the corrected speed to drop below the minimum admissible value (Nc)min or to increase above the maximum admissible value (Nc)max. In some embodiments, if the corrected speed value Nc drops below the minimum or rises above the maximum admissible values, the position of the VSVs can be modified to move the turbocompressor in a different operating point of a different operating envelope.

While the disclosed embodiments of the subject matter described herein have been shown in the drawings and fully described above with particularity and detail in connection with several exemplary embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without materially departing from the novel teachings, the principles and concepts set forth herein, and advantages of the subject matter recited in the appended claims. Hence, the proper scope of the disclosed innovations should be determined only by the broadest interpretation of the appended claims so as to encompass all such modifications, changes, and omissions. In addition, the order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. 

What is claimed is:
 1. A method for regulating a turbocompressor to prevent surge, comprising the following steps: providing at least one surge limit line and/or at least one choke line of said turbocompressor; determining continuously an operating point of the compressor measuring a processing gas temperature at a compressor inlet, the rotary speed of the compressor, a delivery pressure value and a suction pressure value; continuously determining an actual value of a corrected speed of the turbocompressor, the corrected speed being proportional to a ratio of the rotary speed to a square root of the processing gas temperature; continuously determining at least a maximum admissible pressure ratio on said surge limit line and/or at least a minimum admissible pressure ratio on said choke line, corresponding to the actual value of the corrected speed; continuously determining an actual pressure ratio, equal to a ratio between the delivery pressure value and the suction pressure value; and acting upon an antisurge arrangement to recirculate a fraction of a the compressed gas in the compressor through a suction line, if the actual pressure ratio is equal to or higher than the maximum admissible pressure ratio or equal to or lower than the minimum admissible pressure ratio.
 2. The method of claim 1, further comprising the step of calculating a surge parameter defined as a ratio of the maximum admissible pressure ratio to the actual pressure ratio and using said surge parameter to control the antisurge arrangement.
 3. The method of claim 1, further comprising the step of calculating a choke parameter defined as a ratio of the minimum admissible pressure ratio to the actual pressure ratio, and using said choke parameter to control the antichoke arrangement.
 4. The method of claim 1, further comprising the steps of: providing at least one maximum admissible corrected speed and at least one minimum admissible corrected speed of the turbocompressor; if the actual value of the corrected speed is higher than the maximum admissible corrected speed, reducing the rotary speed of the compressor; and if the actual value of the corrected speed is lower than the minimum admissible corrected speed, increasing the rotary speed of the compressor.
 5. The method of claim 1, further comprising the steps of: determining a parameter indicative of a position of variable stator vanes located at the inlet of said turbocompressor; and selecting said surge limit line and/or said choke line as a function of said parameter, among a plurality of surge limit lines and/or a plurality of choke lines, each corresponding to a respective position of the variable stator vanes.
 6. The method of claim 1, further comprising the steps of: setting a maximum admissible corrected speed and a minimum admissible corrected speed corresponding to a position of variable stator vanes located at the inlet of said turbocompressor; and if the actual value of the corrected speed is higher than the maximum admissible corrected speed or if the actual value of the corrected speed is lower than the minimum admissible corrected speed, acting upon the variable stator vanes.
 7. The method of claim 1, further comprising the preceding step of detecting the kind of gas entering in the turbocompressor.
 8. An apparatus for providing an antisurge control for a compression system comprising at least one compressor, said apparatus being arranged and configured for performing a method according to claim
 1. 9. An apparatus for providing an antisurge control for a compression system comprising at least one compressor, said apparatus comprising: a data storage device containing data defining at least one surge limit line and/or at least one choke line of the compressor; an arrangement for determining continuously an operating point of the compressor comprising: a temperature sensor for measuring processing gas temperature at a compressor inlet, a rotary speed sensor for measuring rotary speed of the compressor, and pressure sensors for measuring a delivery pressure value and a suction pressure value; an arrangement for continuously determining an actual value of a corrected speed of the turbocompressor, the corrected speed being proportional to a ratio of the rotary speed to a square root of the processing gas temperature; an arrangement for continuously determining at least a maximum admissible pressure ratio on said surge limit line and/or at least a minimum admissible pressure ratio on said choke line, corresponding to the actual value of the corrected speed; an arrangement for continuously determining an actual pressure ratio, equal to a ratio between the delivery pressure value and the suction pressure value; an antisurge arrangement to recirculate a fraction of compressed gas in the compressor through a the suction line, acted upon if the actual pressure ratio is equal to or higher than the maximum admissible pressure ratio or equal to or lower than the minimum admissible pressure ratio.
 10. The apparatus of claim 9, further comprising devices for calculating a surge parameter defined as a ratio of the maximum admissible pressure ratio to the actual pressure ratio.
 11. The apparatus of claim 9, further comprising devices for calculating a choke parameter defined as a ratio of the maximum admissible pressure ratio to the actual pressure ratio.
 12. The apparatus of claim 9, wherein said storage device contains data defining at least one maximum admissible corrected speed and one minimum admissible corrected speed of said compressor; and wherein a speed control arrangement is provided, which controls a rotary speed of said compressor such that: if the actual value of the corrected speed is higher than the maximum admissible corrected speed, the rotary speed of the compressor is reduced, and if the actual value of the corrected speed is lower than the minimum admissible corrected speed, the rotary speed of the compressor increased.
 13. The apparatus of claim 9, further comprising an actuation device for controlling a position of variable stator vanes of said compressor and wherein said storage device contains data defining a plurality of surge lines and/or choke lines corresponding to a plurality of respective positions of the variable stator vanes.
 14. A compression system comprising at least one compressor and an apparatus according to claim 9, for antisurge-control of said compressor.
 15. The compression system of claim 14, further comprising an arrangement for detecting the kind of gas entering in the turbocompressor.
 16. The apparatus of claim 10, further comprising devices for calculating a choke parameter defined as a ratio of the maximum admissible pressure ratio to the actual pressure ratio. 